Electroless nickel plating of silicone rubber

ABSTRACT

According to the present disclosure, a method for coating nickel on an organosiloxane polymer is provided. A nickel organosiloxane polymer composite is also provided.

TECHNICAL FIELD

The present disclosure relates to a method for coating nickel on anorganosiloxane polymer. The present disclosure also relates to a nickelorganosiloxane polymer composite.

BACKGROUND

Silicone rubber is an elastomer composed of organosiloxane polymers suchthat they contain organic and inorganic moieties. Due to the Si—O bondand inorganic properties, silicone rubber tends to be superior overordinary organic rubbers in terms of flexibility, heat resistance andchemical stability. Consequently, silicone rubber has been used widelyin many areas including electronic, automotive and biomedicalindustries.

The metallization of silicone rubber, i.e. the deposition of thinmetallic film on its surface, is of great interest in the industries.Metalized silicone rubber is a very useful material because it combinesmerits of the metallic surface with those of silicone rubber, such assoftness, flexibility, stretchability, moldability, chemical and heatresistance. Potential applications may therefore include those thatrequire electrical or thermal conductivity, electromagnetic interference(EMI) shielding, actuation, sensing or even corrosion protection. Forexample, metalized silicone rubbers may be used in interconnectors offlexible electronics, in smart sealing components and/or functionalizedmicrofluidic devices.

The metallization of silicone rubber, however, is extremely difficultbecause silicone rubber has very low surface energy and lacks reactivesites.

Conventionally, two types of methods have been reported for metallizingsilicone rubber. One is physical in nature while the other is chemicalin nature.

The physical method relies on metal vapour deposition and may includee-beam assisted deposition, plasma assisted deposition, chemical andphysical vapour deposition etc. Such physical methods, however, likelyrequire high vacuum and tend to be difficult for industry adoption dueto expensive equipments needed. Meanwhile, chemical methods may exploitreduction of metal ions to deposit metal on the polymer, e.g. in aqueoussolution. Such chemical methods may be known as “electroless metalplating”. Examples so far include coating silicone rubber with noblemetals such as platinum (Pt) and gold (Au) via electroless plating, andin such examples, the silicone rubber surface is first activated byultraviolet (UV) laser and H₂PtCl₆ solution, followed by coating withthe noble metal salt solution that may be highly reactive. Although suchchemical methods have been extensively utilized due to their energyefficiency, expensive specialized equipments and rare materials rendersuch chemical methods economically unfeasible.

Conventionally, electroless nickel plating has been specifically appliedon silicone-rich polyester surface but not on neat organosiloxanepolymers (e.g. neat silicone rubber surface) due to inertness and lowsurface energy of the latter. The term “neat” means that a material issolely made of a single material. For example, a neat silicone rubber iscomposed only of silicone rubber.

There is thus a need to provide for a method of electroless nickelplating on organsiloxane polymers, including silicone rubber, whichresolves and/or ameliorates the issues mentioned above. The methodprovided should serve as an improved way to coat nickel and/or nickelderivatives onto neat organosiloxane polymers, including siliconerubber.

There is also a need to provide for an organosiloxane polymer compositewith nickel and/or nickel derivatives coated thereon. The nickel coatedorganosiloxane polymer composite should at least circumvent orameliorate one or more of the issues as mentioned above.

SUMMARY

In one aspect, there is provided for a method for coating nickel on anorganosiloxane polymer comprising:

forming a transition metal oxide on the organosiloxane polymer;

etching the transition metal oxide with a basic solution;

contacting the organosiloxane polymer comprising the etched transitionmetal oxide with an aqueous solution comprising a positively chargedspecies to attach the positively charged species on the etchedtransition metal oxide;

depositing a metal catalyst on the positively charged species; and

treating the metal catalyst with an acidic solution to develop anactivated organosiloxane polymer before transferring the activatedorganosiloxane polymer to a solution comprising nickel and/or nickelderivatives.

In another aspect, there is provided for a nickel organosiloxane polymercomposite comprising:

a transition metal oxide layer formed on the organosiloxane polymer; and

a positively charged species attached on the transition metal oxidelayer with nickel coated on the positively charged species.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are not necessarily to scale, emphasis instead generallybeing placed upon illustrating the principles of the invention. In thefollowing description, various embodiments of the present disclosure aredescribed with reference to the following drawings, in which:

FIG. 1 shows a schematic illustration of an embodiment of the presentmethod.

FIG. 2 shows the contact angle (CA) of water droplet on various siliconerubber surfaces.

FIG. 3 shows photos of untreated silicone rubber and silicone rubberthat has undergone electroless nickel plating according to embodimentsof the present method.

FIG. 4 shows scanning electron microscopy (SEM) images of a nickelplated silicone rubber derived from an embodiment of the present method.The scale bar in both images represent 10 μm. The difference between theleft and right SEM images lies in their magnification. The left SEMimage has a magnification of ×300 while the right SEM image has amagnification of ×2500.

FIG. 5 shows a schematic diagram of coating silicone with TiO₂ usingtitanium isopropoxide (TIP) and isopropyl alcohol (IPA) according to anembodiment disclosed herein.

FIG. 6 shows the negatively charged structure of a tin-palladium (Sn—Pd)colloidal catalyst.

FIG. 7 shows a typical setup of electroless nickel plating (ENP) bath.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific details and embodiments inwhich the invention may be practised.

Features that are described in the context of an embodiment maycorrespondingly be applicable to the same or similar features in theother embodiments. Features that are described in the context of anembodiment may correspondingly be applicable to the other embodiments,even if not explicitly described in these other embodiments.Furthermore, additions and/or combinations and/or alternatives asdescribed for a feature in the context of an embodiment maycorrespondingly be applicable to the same or similar feature in theother embodiments.

The present disclosure relates to a method that enables the coating ofnickel onto an organosiloxane polymer (e.g. silicone rubber) by usingelectroless nickel plating (ENP). This generally involves modifying theorganosiloxane polymer's surface so that it becomes hydrophilic andpositively charged. On this modified surface, palladium (Pd) catalyst isthen deposited with subsequent ENP achieved. The present disclosure alsorelates to a nickel organosiloxane polymer composite derived from thepresent method. Advantageously, the nickel organosiloxane polymercomposite has improved adhesion strength of nickel plated on theorganosiloxane polymer with improved plating quality.

Electroless plating is a process in which a metal layer is deposited ona substrate by chemical reduction in the absence of an external electriccurrent. This process is advantageously energy efficient.Conventionally, platinum plating on silicone rubber and nickel platingon silicone-rich polyester have been attempted. However, these requirespecialized equipments, such as ultraviolet laser or argon plasma totreat the silicone rubber surface, which render such techniqueseconomically unviable. The present method, however, uses a solutionbased method to modify an organosiloxane polymer (e.g. silicone rubber)surface for deposition of a catalyst for subsequent electroless nickelplating. The advantages of the present method are therefore, an easy toapply method, improved electroless nickel plating efficiency withreduced processing time of less than 30 mins, cost effectiveness, andthe resultant nickel being plated in high quality.

Having outlined various advantages of the present method and the nickelorganosiloxane polymer composite, definitions of certain terms are firstdiscussed before going into details of the various embodiments.

The word “substantially” does not exclude “completely” e.g. acomposition which is “substantially free” from Y may be completely freefrom Y. Where necessary, the word “substantially” may be omitted fromthe definition of the invention.

In the context of various embodiments, the articles “a”, “an” and “the”as used with regard to a feature or element include a reference to oneor more of the features or elements.

In the context of various embodiments, the term “about” or“approximately” as applied to a numeric value encompasses the exactvalue and a reasonable variance.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items.

As used herein, the phrase of the form of “at least one of A and B” mayinclude A or B or both A and B. Correspondingly, the phrase of the formof “at least one of A and B and C”, or including further listed items,may include any and all combinations of one or more of the associatedlisted items.

Unless specified otherwise, the terms “comprising” and “comprise”, andgrammatical variants thereof, are intended to represent “open” or“inclusive” language such that they include recited elements but alsopermit inclusion of additional, unrecited elements.

Having defined the various terms as mentioned above, details of thevarious embodiments are now described below.

In the present disclosure, there is provided for a method for coatingnickel on an organosiloxane polymer comprising: forming a transitionmetal oxide on the organosiloxane polymer, etching the transition metaloxide with a basic solution, contacting the organosiloxane polymercomprising the etched transition metal oxide with an aqueous solutioncomprising a positively charged species to attach the positively chargedspecies on the etched transition metal oxide, depositing a metalcatalyst on the positively charged species, and treating the metalcatalyst with an acidic solution to develop an activated organosiloxanepolymer before transferring the activated organosiloxane polymer to asolution comprising nickel and/or nickel derivatives.

In various embodiments, the organosiloxane polymer may be selected fromthe group consisting of polydimethylsiloxane, vinyl methyl polysiloxane(VMQ), phenyl methyl polysiloxane (PMQ), phenyl vinyl methylpolysiloxane (PVMQ), fluoro vinyl methyl polysiloxane (FVMQ) andderivatives of silicone rubber, wherein Q represents a quaternarysilicon. In other words, the organosiloxane polymer may be selected fromthe group consisting of polydimethylsiloxane, vinyl methyl polysiloxane,phenyl methyl polysiloxane, phenyl vinyl methyl polysiloxane, fluorovinyl methyl polysiloxane and derivatives of silicone rubber, whereinthe polysiloxane may comprise at least one quaternary silicon. Toillustrate, the silicon atoms in vinyl methyl polysiloxane may bequaternary silicon atoms.

In various embodiments, the transition metal oxide may comprise titaniumoxide. In various embodiments, the transition metal oxide may beselected from the group consisting of titanium oxide, zirconium oxide,vanadium oxide, hafnium oxide, niobium oxide and tantalum oxide.

According to the present method, the forming may comprise contacting theorganosiloxane polymer with a solution comprising a swelling agent and atransition metal oxide precursor for up to 30 minutes, and drying theorganosiloxane polymer in ambient conditions comprising a temperaturefrom 15° C. to 30° C. and a relative humidity from 30% to 70% to formthe transition metal oxide on the organosiloxane polymer.

The transition metal oxide precursor may be used to form an intermediatelayer linking the organosiloxane polymer (e.g. silicone rubber) with thesubsequent transition metal oxide layer that may be formed on surface ofthe organosiloxane polymer. The transition metal oxide precursor mayhave one or more alkoxide groups that can be hydrolyzed to form athree-dimensional (3D) network. In some embodiments, the transitionmetal oxide precursor may have four alkoxide groups. When theorganosiloxane polymer (e.g. silicone rubber) is contacted with thesolution comprising the swelling agent and transition metal oxideprecursor, the organosiloxane polymer may swell, i.e. the organosiloxanepolymer may expand to allow diffusion of the swelling agent as well astransition metal oxide precursor into the matrix of the organosiloxanepolymer. The hydrolyzation and/or gelation of the transition metal oxideprecursor embedded into and/or on the organosiloxane polymer may form atransition metal oxide network that mixes or merges (interpenetrating)with matrix of the organosiloxane polymer. The existence of such anintermediate layer in a transition metal oxide-polysiloxaneinterpenetrating structure advantageously enhances adhesion between thetransition metal oxide layer and organosiloxane polymer. The thicknessand structure of this intermediate layer may depend on time,temperature, humidity etc. For example, if the time of contact is shortand humidity is low, the intermediate layer developed may be too thin topromote adhesion. Intermediate layer with high thickness, however,undesirably reduces flexibility of the organosiloxane polymer. Theadvantage to the duration, temperature and/or humidity range lies in theability to form an intermediate layer with a thickness sufficient toenhance adhesion between the transition metal oxide layer andorganosiloxane polymer.

In some embodiments, the organosiloxane polymer may be contacted withthe solution comprising the swelling agent and the transition metaloxide precursor for up to 30 mins, up to 20 mins, up to 10 mins, up to 5minutes etc. The longer the duration, the higher the amount oftransition metal oxide that may be eventually formed on theorganosiloxane polymer. The humidity (i.e. moisture in air) allowshydrolysis of the transition metal oxide precursor to form transitionmetal oxide on the organosiloxane polymer.

In various embodiments, the swelling agent may be selected from thegroup consisting of isopropyl alcohol, methanol, 2-methoxyethanol,ethanol, 1-propanol, tert-butanol and their mixtures thereof. Theswelling agent may penetrate the matrix of the organosiloxane polymer sothat the transition metal oxide precursor may be able to diffuse intothe matrix. If the transition metal oxide precursor only absorbs on thesurface of the organosiloxane polymer, poor adhesion between thesubsequently formed transition metal oxide and the organosiloxanepolymer may develop. The swelling agent helps in formation of anintermediate layer to enhance such an adhesion. The organosiloxanepolymer may not be swellable when the solution only contains transitionmetal oxide precursor.

In various embodiments, the swelling agent may comprise or consist of analcohol. The use of alcohol as the swelling agent is advantageousbecause it is compatible with the transition metal oxide precursor anddoes not compromise hydrolysis rate of the transition metal oxideprecursor. The alcohol may be isopropyl alcohol. In some embodiments,the swelling agent may comprise or consist of isopropyl alcohol.Isopropyl alcohol may be preferred as it provides adequate swelling ofthe organosiloxane polymer and easily removed due to its low boilingpoint.

In various embodiments, the transition metal oxide precursor maycomprise or consist of titanium isopropoxide. In some embodiments, thetransition metal oxide precursor may be selected from the groupconsisting of titanium isopropoxide, titanium propoxide, titaniummethoxide, titanium ethoxide, titanium butoxide, titanium tert-butoxide,titanium 2-ethylhexyloxide, zirconium tert-butoxide, zirconiumisopropoxide, vanadium isobutoxide, vanadium oxytriethoxide, vanadiumoxytriisopropoxide, vanadium oxytripropoxide, hafnium n-butoxide,hafnium tert-butoxide, niobium ethoxide, tantalum methoxide, tantalumethoxide and tantalum butoxide.

In various embodiments, the transition metal oxide prescursor and theswelling agent may comprise a volume ratio of 1:1 to 1:99. In someinstances, the volume ratio may be 1:9 (e.g. a TIP/IPA volume ratio of10:90). In other instances, the volume ratio may be 1:3 (e.g. a TIP/IPAvolume ratio of 25:75).

After forming the transition metal oxide on the organosiloxane polymer,the transition metal oxide may be etched to modify hydrophilicity of thetransition metal oxide coated organosiloxane polymer (e.g. siliconerubber) such that there is an increase in surface energy. Etching atthis stage of the present method advantageously removes contaminantsthat may be present on surface of the modified organosiloxane polymer.This also advantageously increases surface energy of the modifiedorganosiloxane polymer and may lead to stronger absorption and/oradsorption of positively charges species that may be subsequently coatedthereon. In various embodiments, the etching may occur for up to 1 min.In some embodiments, the etching may occur for at least 30 seconds orless than 30 seconds.

To carry out the etching, a basic solution may be used. The basicsolution may comprise or consist of NaOH according to variousembodiments. The basic solution may comprise a concentration of 0.1 to10 M in various embodiments. In some embodiments, the basic solution maybe selected from the group consisting of NaOH, KOH, LiOH and ammonia.Where NaOH is used, the NaOH may comprise or consist of a concentrationof 3 M.

After etching the transition metal oxide, the organosilioxane polymer(e.g. silicone rubber) may be coated with a positively charged speciessuch that the positively charged species attached on the etchedtransition metal oxide changes polarity of charges present on the etchedtransition metal oxide. By using a positively charged species, thenegatively charged surface of the organosiloxane polymer coated with theetched transition metal oxide may be tuned to become positively charged.This advantageously improves deposition of catalyst for subsequentelectroless metal plating. The positively charged species is not asurfactant. The positively charged species may carry multiple charges oneach molecule of the species. When they are absorbed and/or adsorbed onthe negatively charged transition metal oxide, some of the charges maybe neutralized while the rest remain charged. Therefore, the chargingstate of the surface may be inverted from negative to positive. Incomparison, a surfactant molecule tends to carry only one charge and isunable to invert the charge of the transition metal oxide layer.

In various embodiments, the positively charged species may be coated onthe etched transition metal oxide by contacting the organosiloxanepolymer with an aqueous solution comprising the positively chargedspecies. In various embodiments, the aqueous solution may comprise orconsist of 0.01 weight percent (wt %) to 1 wt % of the positivelycharged species.

In various embodiments, the positively charged species may be selectedfrom the group consisting of positively charged nanoparticles ornanocolloids, and dendrimers comprising nitrogen. The positively chargedspecies may also be selected from the group consisting of polymershaving at least one primary amine, secondary amine, tertiary amine,pyridinyl, quaternized amine and/or quaternized pyridinyl, and theirmixtures thereof. The positively charged nanoparticles or nanocolloidsmay comprise or consist of cetyltrimethylammonium bromide (CTAB)stabilized gold nanoparticles, spermidine stabilized silvernanoparticles, 2-(dimethylamino)ethanethiol-capped CdTe quantum dotsand/or their mixtures thereof.

The dendrimers may comprise or consist of polyamidoamine dendrimers,polyethylenimine dendrimers, polypropylenimine hexadecaamine dendrimers,ammonium-capped thiophosphoryl chloride dendrimers, ammonium-cappedcyclotriphosphazene dendrimers and/or their mixtures thereof. Thepolymers may comprise or consist of polyamine, polyallylamine,polyetheramine, polyethylenimine, polyvinylpyridine, polybrene,chitosan, poly(2-(trimethylamino)ethyl methacrylate),poly(diallyldimethylammonium chloride),poly(N,N-dimethyl-3,5-dimethylene-piperidinium chloride),poly(2-vinyl-1-methylpyridinium bromide) and/or their mixtures thereof.In some embodiments, the positively charged species may bepolyethylenimine.

In various embodiments, the organosiloxane polymer of the contactingstep may be contacted with the aqueous solution (comprising thepositively charged species) for at least 5 minutes. In otherembodiments, the contacting may be 5 mins or less.

Once the positively charged species are attached on the etchedtransition metal oxide of the organosiloxane polymer, the catalyst forENP may be deposited thereon. In the present method, the depositing maycomprise or consist contacting the positively charged species with asolution comprising the metal catalyst for at least 5 minutes to depositthe metal catalyst on the positively charged species. In otherembodiments, the duration may be 5 mins or less.

According to various embodiments, the metal catalyst may comprise orconsist of, without being limited to, tin-palladium (Sn—Pd) colloidalmetal catalyst. Other catalyst may be used depending on the metal to bedeposited. Pd based catalyst may be used for plating other metals. Othercatalysts, such as those that are copper based, may be used for platingother metals like gold, platinum etc.

In the present method, the metal catalyst (e.g. the tin-palladiumcolloidal metal catalyst) may be prepared by: dissolving PdCl₂ in anamount of 0.05 gram to 0.15 gram in 30 ml to 50 ml of HCl to form afirst solution, dissolving SnCl₂ in 10 ml to 30 ml of HCl andsubsequently adding 10 ml to 30 ml of water to form a second solution,mixing the first and second solutions, and continuously stirring themixture (of the first and second solutions) for 10 minutes to 30minutes, dissolving 4 gram to 5 gram of NaCl, 0.4 gram to 1.2 gram ofNaSnO₃ and 5 gram to 15 gram of SnCl₂ in 40 ml to 60 ml of water to forma third solution, adding the third solution to the mixture over aduration of 20 minutes to 40 minutes, and incubating the mixture undercontinuous stirring for 3 hours to 5 hours at 60° C. to 70° C. Theamount of each reagents used and the timing affect the quality of theSn—Pd catalyst formed. The Sn—Pd catalyst formed under such conditionsis advantageously stable with high catalytic activity, uniform in sizeand in the nano-scale. Otherwise, the Sn—Pd colloidal catalyst tends toaggregate during or after its formation, and/or catalytic activity maybe reduced.

In some embodiments, the SnCl₂ for forming the second solution may bedissolved in an amount that may be at least three times the amount ofPdCl₂ dissolved for forming the first solution. In some embodiments, theSnCl₂ may be dissolved in an amount of 0.5 gram to 1.5 gram.

In various embodiments, the concentration of HCl for forming the firstsolution and second solution may be 4 to 8 M and 9 to 12 M,respectively. In some embodiments, the concentration of HCl for formingthe first solution and second solution may be 6 M and 12 M,respectively. These concentrations of HCl is advantageously useful forcontrolling size of the Sn—Pd nanocolloidal catalysts. Otherwise, thecatalyst particle size may become undesirably large until it adverselyaffects their deposition on the modified organosiloxane polymer and alsothe catalytic activity.

Once the metal catalyst has been prepared and deposited on thepositively charged species of the organosiloxane polymer, the metalcatalyst may be treated with an acidic solution as mentioned above. Thismay activate the metal catalyst, so that the metal (e.g. nickel) can beelectrolessly plated on. This treatment may be for at least 1 min oreven less than 1 min. In some embodiments, the treatment with the acidicsolution may be 3 mins. The acidic solution may comprise or consist ofHCl. Since the outer layer of a Sn—Pd colloidal catalyst may containSn²⁺ and/or Cl⁻, activation using an acidic solution may be required toremove such ions to have the active Pd site(s) exposed.

Once the metal catalyst has been deposited and activated, the activatedorganosiloxane polymer may be transferred to a solution bath forelectroless metal plating, particularly electroless nickel plating(ENP). In various embodiments, the solution may comprise nickel and/ornickel derivatives. Such a solution may be an electroless nickel plating(ENP) solution. The activated organosiloxane polymer may be contactedwith the ENP solution for at least 2 minutes. In other embodiments, thecontact may be 2 mins or less.

In various embodiments, the ENP solution of the present method may beprepared by mixing 4 to 6 g/l (gram/litre) of NiSO₄, 0.5 to 1.5 g/l ofNaH₂PO₄, 0.5 to 1.5 g/l of oxycarboxylic acid and an amount of ammoniasufficient to maintain pH of the ENP solution between 4.5 and 5.5. Theamounts for the various reagents may be determined to provide apreferred plating speed of the metal, in this instance, nickel. Ifconcentrations happen to be too high, plating may be too fast andquality of the metal (e.g. nickel) coating may be adversely affected.The pH may be critical in the redox-reduction reaction between Ni²⁺ andH₂PO₄ ⁻. Otherwise, the reaction for electroless plating would notoccur.

In some embodiments, the ENP solution may be prepared by mixing 4.7 g/lof NiSO₄, 1 g/l of NaH₂PO₄, 1 g/l of oxycarboxylic acid and an amount ofammonia sufficient to maintain pH of ENP solution at 4.9.

In various embodiments, the nickel derivatives may comprise or consistof nickel sulphate. The nickel derivatives may be selected from thegroup consisting of nickel sulphate, nickel chloride, nickel acetate,nickel nitrate and any acceptable nickel salts thereof.

In the present method, a 1-part silicone rubber or a 2-part siliconerubber may be used as the organosiloxane polymer. To prepare suchsilicone rubbers, the forming step of the present method may furthercomprise preparation of the organosiloxane polymer, the preparationcomprising the steps of: blending an organosiloxane polymer precursorwith a cross-linking agent to form a blended mixture and curing theblended mixture at 170° C. to 180° C. under 8 MPa to 12 MPa for at least5 minutes, or mixing two organosiloxane polymer precursors with a curingagent and curing at 25° C. to 35° C. for 48 hours. In variousembodiments, the cross-linking agent may be dicumyl peroxide. The curingagent may comprise or consist of platinum.

Between various steps of the present method, washing off residualchemicals may be required. In various embodiments, the method mayfurther comprise rinsing the organosiloxane polymer with isopropylalcohol before the etching and rinsing with water after each of theetching, the contacting, the depositing and the treating.

The present disclosure also provides for a nickel organosiloxane polymercomposite comprising: a transition metal oxide layer formed on theorganosiloxane polymer, and a positively charged species attached on thetransition metal oxide layer with nickel coated on the positivelycharged species. The nickel organosiloxane polymer composite, derivedfrom the present method, has a high quality of nickel plated on theorganosiloxane polymer with improved adhesion of nickel to theorganosiloxane polymer. Embodiments and advantages described above inrelation to the present method may be applicable and/or valid toembodiments of the nickel organosiloxane polymer composite, and viceversa.

In various embodiments, the nickel organosiloxane polymer may furthercomprise a trace amount of Sn—Pd catalyst under the nickel coated on thepositively charged species. This may help to improve adhesion of nickelto the organosiloxane polymer.

The organosiloxane polymer may be selected from the group consisting ofpolydimethylsiloxane, vinyl methyl polysiloxane, phenyl methylpolysiloxane, phenyl vinyl methyl polysiloxane, fluoro vinyl methylpolysiloxane and derivatives of silicone rubber, wherein thepolysiloxane comprises a quaternary silicon.

In various embodiments, the transition metal oxide layer may comprise orconsist of titanium oxide. In various embodiments, the transition metaloxide layer may be selected from the group consisting of titanium oxide,zirconium oxide, vanadium oxide, hafnium oxide, niobium oxide andtantalum oxide.

The positively charged species may be selected from the group consistingof positively charged nanoparticles or nanocolloids, and dendrimerscomprising nitrogen. The positively charged species may also be selectedfrom the group consisting of polymers having at least one primary amine,secondary amine, tertiary amine, pyridinyl, quaternized amine and/orquaternized pyridinyl, and their mixtures thereof. The positivelycharged nanoparticles or nanocolloids may comprise or consist ofcetyltrimethylammonium bromide (CTAB) stabilized gold nanoparticles,spermidine stabilized silver nanoparticles,2-(dimethylamino)ethanethiol-capped CdTe quantum dots and/or theirmixtures thereof. The dendrimers may comprise or consist ofpolyamidoamine dendrimers, polyethylenimine dendrimers,polypropylenimine hexadecaamine dendrimers, ammonium-cappedthiophosphoryl chloride dendrimers, ammonium-capped cyclotriphosphazenedendrimers and/or their mixtures thereof. The polymers may comprise orconsist of polyamine, polyallylamine, polyetheramine, polyethylenimine,polyvinylpyridine, polybrene, chitosan, poly(2-(trimethylamino)ethylmethacrylate), poly(diallyldimethylammonium chloride),poly(N,N-dimethyl-3,5-dimethylene-piperidinium chloride),poly(2-vinyl-1-methylpyridinium bromide) and/or their mixtures thereof.In some embodiments, the positively charged species may bepolyethylenimine.

In various embodiments, the nickel, the Sn—Pd catalyst, the positivelycharged species and the transition metal oxide layer may form or maycomprise a thickness of 2.52 to 3 μm. The overall thickness of theselayers may also depend on the various agents used and duration of eachof the steps of the present method.

While the methods described above are illustrated and described as aseries of steps or events, it will be appreciated that any ordering ofsuch steps or events are not to be interpreted in a limiting sense. Forexample, some steps may occur in different orders and/or concurrentlywith other steps or events apart from those illustrated and/or describedherein. In addition, not all illustrated steps may be required toimplement one or more aspects or embodiments described herein. Also, oneor more of the steps depicted herein may be carried out in one or moreseparate acts and/or phases.

EXAMPLES

The present disclosure relates to an electroless plating method to coatnickel on an organosiloxane polymer e.g. silicone rubber,polydimethylsiloxane (PDMS), vinyl methyl polysiloxane (VMQ), phenylmethyl polysiloxane (PMQ), phenyl vinyl methyl polysiloxane (PVMQ),fluoro vinyl methyl polysiloxane (FVMQ) and derivatives of siliconerubber, wherein Q represents a quaternary silicon. The latter means thatthe silicon atom(s) in the polysiloxane (with Q in its abbreviation) isa quaternary silicon atom(s).

The present method may include, as a non-limiting example, the followingsteps: sol-gel coating of titanium oxide (TiO₂) on silicone surface,etching of TiO₂ layer to increase the hydrophilicity and/or surfaceenergy, coating of positively charged polyethylenimine, deposition oftin-palladium (Sn—Pd) colloidal catalyst, activation and electrolessnickel plating (ENP). By using TiO₂ coating and then treatment withpolyethylenimine (PEI), the silicone surface may be tuned from beinginert and hydrophobic to hydrophilic and positively charged. The latterallows for deposition of Pd catalyst, which aids in electroless platingof nickel. The nickel film obtained via such steps advantageously hasgood continuity, high electrical conductivity and strong adhesion withthe silicone.

The present method involves plating metal on silicone. The presentmethod also involves modifying silicone surface to be hydrophilic andcharged. The present method further involves depositing catalyst onsilicone surface for electroless plating.

According to various non-limiting embodiments of the present method, ametalized coating on silicone rubber that may provide high electricalconductivity and good durability may be produced. The present method andnickel coated organosiloxane polymer composite are described below byway of non-limiting examples.

Example 1a: Schematic Illustration of the Present Method

The present method of ENP is schematically illustrated in FIG. 1, whichincludes the following steps.

In step 100, silicone rubber 1 was dipped into titaniumisopropoxide-isopropyl alcohol (TIP/IPA) mixture (1-75 vol %) for about5 minutes (mins) to coat or anchor a thin TiO₂ film 3 on the siliconerubber surface. This may be called a sol-gel procedure.

In step 200, the TiO₂ modified silicone rubber from step 100 was dippedinto aqueous NaOH (0.1-10 M) for about 1 min so as to enhancehydrophilicity or increase surface energy of the TiO₂ film 3 bychemically etching it, and to increase its electrical potential. Anegatively charged surface 5 was obtained.

In step 300, the negatively charged silicone rubber from step 200 wasdipped into aqueous PEI (0.01-1 wt %) for about 10 mins to invert thesign of charge of the surface 5 by adsorption of positively charged PEI7. A positively charged surface 9 was then obtained.

In step 400, the positively charged silicone rubber from step 300 wasdipped into a Sn—Pd colloidal suspension solution for about 5 mins todeposit Pd catalyst 11.

In step 500, the catalyzed silicone rubber from step 400 was dipped intoHCl solution for about 3 mins to activate the Pd catalyst 11 and nickel13 was subsequently coated on the silicone rubber surface via ENP forabout 5 mins.

The durations exemplified for each step disclosed in this example arenon-limiting and may be shorter.

Example 1b: Nickel Plated Silicone Rubber with Shore a Hardness of 30

Silicone rubber with Shore A hardness of 30 was prepared by blending1-part silicone (from Momentive) with dicumyl peroxide and cured at 175°C. under a pressure of 10 MPa for 6 mins. The coating was performedthrough the following steps:

(1) The silicone rubber was immersed in TIP/IPA solution (50:50 volumeratio) for 5 mins and then rinsed with IPA and dried in ambient air.

(2) The modified silicone rubber from (1) was immersed in 3 M NaOHsolution for 1 min and then rinsed with water.

(3) The modified silicone rubber from (2) was immersed in 1 weightpercent (wt %) branched PEI solution for 10 mins and then rinsed withwater.

(4) The modified silicone rubber from (3) was immersed in Sn—Pdcolloidal catalyst solution for 5 mins. This was followed by rinsingwith water.

(5) The modified silicone rubber from (4) was immersed in 1 M HCl for 3mins and rinsed with water, then immersed in an ENP solution at 89° C.for 5 mins with the following composition: 4.7 g/l (gram/litre) ofNiSO₄, 1 g/l of NaH₂PO₂, 1 g/l of oxycarboxylic acid and a certainamount of ammonia to adjust the pH to 4.9.

The Sn—Pd catalyst of step (4) was prepared through the following steps:

(a) Dissolving 0.1 g PdCl₂ in 40 ml of 6 M HCl under magnetic stirring.

(b) Adding 1 g SnCl₂ into 20 ml of 12 M HCl. After complete dissolution,20 ml of water was added to the solution of (b) to become more diluted.

(c) Solution of (b) was then added to solution of (a) under continuousstirring for 20 mins.

(d) 4.4 g of NaCl, 0.8 g of NaSnO₃ and 10 g of SnCl₂ were dissolved in50 ml water. Subsequently, solution of (d) was added slowly to solutionof (c) for 30 mins.

(e) The mixture from (d) was kept in a water bath at 65° C. for 4 hoursunder stirring.

Example 1c: Nickel Plated Silicone Rubber with Shore A Hardness of 70

Silicone rubber with Shore A hardness of 70 was prepared by blending1-part silicone (from Momentive) with dicumyl peroxide and cured at 175°C. under 10 MPa for 6 mins. The coating was performed through thefollowing steps:

(1) The silicone rubber was immersed in TIP/IPA solution (50:50) for 5mins and then rinsed with IPA and dried in ambient air.

(2) The modified silicone rubber from (1) was immersed in 3 M NaOHsolution for 1 min and then rinsed with water.

(3) The modified silicone rubber from (2) was immersed in 1 wt %branched PEI solution for 10 mins and then rinsed with water.

(4) The modified silicone rubber from (3) was immersed in Sn—Pdcolloidal catalyst solution for 5 mins, followed by rinsing with water.

(5) The modified silicone rubber from (4) was immersed in 1 M HCl for 3mins and rinsed with water, and then immersed in an ENP solution at 89°C. for 5 mins with the following composition: 4.7 g/l of NiSO₄, 1 g/l ofNaH₂PO₂, 1 g/l of oxycarboxylic acid and a certain amount of ammonia toadjust the pH to 4.9.

The Sn—Pd catalyst of step (4) was prepared through the following steps:

(a) 0.1 g PdCl₂ was dissolved in 40 ml of 6 M HCl under magneticstirring.

(b) 1.0 g SnCl₂ was added into 20 ml of 12 M HCl. After completedissolution, 20 ml water was added to the solution.

(c) Solution of (b) was added into solution of (a) under continuousstirring for 20 mins.

(d) 4.4 g of NaCl, 0.8 g of NaSnO₃ and 10 g of SnCl₂ were dissolved in50 ml water. Subsequently, solution of (d) was added slowly intosolution of (c) for 30 mins.

(e) The mixture from (d) was kept in a water bath at 65° C. for 4 hoursunder stirring.

Example 1d: Nickel Plated Silicone Rubber with Shore a Hardness of 43

Silicone rubber with Shore A hardness of 43 was prepared from 2-partssilicone (Sylgard 184). The 2-parts silicones comprise platinum (Pt)curing agent which was first blended and then cured at room temperaturefor 48 hours. The coating was performed through the following steps:

(1) The 2-parts silicone rubber was immersed in TIP/IPA (50:50) solutionfor 5 mins and then rinsed with IPA and dried in ambient air.

(2) The modified silicone rubber from (1) was immersed in 3 M NaOHsolution for 1 min and then rinsed with water.

(3) The modified silicone rubber from (2) was immersed in 1 wt %branched PEI solution for 10 mins and then rinsed with water.

(4) The modified silicone rubber from (3) was immersed in Sn—Pdcolloidal catalyst for 5 mins, followed by rinsing with water.

(5) The modified silicone rubber from (4) was immersed in 1 M HCl for 3mins and rinsed with water, and then immersed in an ENP solution at 89°C. for 5 mins with the following composition: 4.7 g/l of NiSO₄, 1 g/l ofNaH₂PO₂, 1 g/l of oxycarboxylic acid and a certain amount of ammonia toadjust the pH to 4.9.

The Sn—Pd catalyst of step (4) was prepared through the following steps:

(a) 0.1 g PdCl₂ was dissolved in 40 ml of 6 M HCl under magneticstirring.

(b) 1.0 g SnCl₂ was added into 20 ml of 12 M HCl. After completedissolution, 20 mL water was added to the solution.

(c) Solution of (b) was added into solution of (a) under continuousstirring for 20 mins

(d) 4.4 g of NaCl, 0.8 g of NaSnO₃ and 10 g of SnCl₂ were dissolved in50 ml water. Subsequently, solution of (d) was added slowly intosolution of (c) for 30 mins.

(e) The mixture from (d) was kept in a water bath at 65° C. for 4 hoursunder stirring.

Comparative Example 1a: Present Method without TiO₂ Coating

Silicone rubber with Shore A hardness of 30 was prepared by blending1-part silicone (from Momentive) with dicumyl peroxide and cured at 175°C. under 10 MPa for 6 mins. The coating was performed through thefollowing steps:

(1) The silicone rubber was immersed in 3 M NaOH solution for 1 min andthen rinsed with water.

(2) The modified silicone rubber from (1) was immersed in 1 wt %branched PEI solution for 10 mins and then rinsed with water.

(3) The modified silicone rubber from (2) was immersed in Sn—Pdcolloidal catalyst for 5 mins, followed by rinsing with water.

(4) The modified silicone rubber from (3) was immersed in 1 M HCl for 3mins and rinsed with water, and then immersed in an ENP solution at 89°C. for 5 mins with the following composition: 4.7 g/l of NiSO₄; 1 g/l ofNaH₂PO₂, 1 g/l of oxycarboxylic acid and a certain amount of ammonia toadjust the pH to 4.9.

The Sn—Pd catalyst of step (3) was prepared through the following steps:

(a) 0.1 g PdCl₂ was dissolved in 40 ml of 6 M HCl under magneticstirring.

(b) 1.0 g SnCl₂ was added into 20 ml of 12 M HCl. After completedissolution, 20 ml water was added to the solution.

(c) Solution of (b) was added into solution of (a) under continuousstirring for 20 mins.

(d) 4.4 g of NaCl, 0.8 g of NaSnO₃ and 10 g of SnCl₂ were dissolved in50 ml water. Subsequently, solution of (d) was added slowly intosolution of (c) for 30 mins.

(e) The mixture from (d) was kept in a water bath at 65° C. for 4 hoursunder stirring.

Comparative Example 1b: Present Method without NaOH Etching

Silicone rubber with Shore A hardness of 30 was prepared from byblending 1-part silicone (from Momentive) with dicumyl peroxide andcured at 175° C. under 10 MPa for 6 mins. The coating is performedthrough the following steps:

(1) The silicone rubber was immersed in TIP/IPA solution (50:50) for 5mins and then rinsed with IPA and dried in ambient air.

(2) The modified silicone rubber from (1) was immersed in 1 wt %branched polyethylenimine (PEI) solution for 10 min, then rinsed withwater.

(3) The modified silicone rubber from (2) was immersed in Sn—Pdcolloidal catalyst for 5 mins, followed by rinsing with water.

(4) The modified silicone rubber from (3) was immersed in 1 M HCl for 3mins and rinsed with water, and then immersed in an ENP solution at 89°C. for 5 mins with the following composition: 4.7 g/l of NiSO₄, 1 g/l ofNaH₂PO₂, 1 g/l of oxycarboxylic acid and a certain amount of ammonia toadjust the pH to 4.9.

The Sn—Pd catalyst of step (3) was prepared through the following steps:

(a) 0.1 g PdCl₂ was dissolved in 40 ml of 6 M HCl under magneticstirring.

(b) 1.0 g SnCl₂ was added into 20 ml of 12 M HCl. After completedissolution, 20 ml water was added to the solution.

(c) Solution of (b) was added into solution of (a) under continuousstirring for 20 mins.

(d) 4.4 g of NaCl, 0.8 g of NaSnO₃ and 10 g of SnCl₂ were dissolved in50 ml water. Subsequently, solution of (d) was added slowly intosolution of (c) for 30 mins.

(e) The mixture from (d) was kept in a water bath at 65° C. for 4 hoursunder stirring.

Comparative Example 1c: Present Method without PEI Coating

Silicone rubber with Shore A hardness of 30 was prepared by blending1-part silicone (from Momentive) with dicumyl peroxide and cured at 175°C. under 10 MPa for 6 mins. The coating was performed through thefollowing steps:

(1) The silicone rubber was immersed in TIP/IPA solution (50:50) for 5mins and then rinsed by IPA and dried in ambient air.

(2) The modified silicone rubber from (1) was immersed in 3 M NaOHsolution for 1 min and then rinsed with water.

(3) The modified silicone rubber from (2) was immersed in Sn—Pdcolloidal catalyst for 5 mins, followed by rinsing with water.

(4) The modified silicone rubber from (3) was immersed in 1 M HCl for 3mins and rinsed with water, and then immersed in an ENP solution at 89°C. for 5 mins with the following composition: 4.7 g/l of NiSO₄, 1 g/l ofNaH₂PO₂, 1 g/l of oxycarboxylic acid and a certain amount of ammonia toadjust the pH to 4.9.

The Sn—Pd catalyst of step (3) was prepared through the following steps:

(a) 0.1 g PdCl₂ was dissolved in 40 ml of 6 M HCl under magneticstirring.

(b) 1.0 g SnCl₂ was added into 20 ml of 12 M HCl. After completedissolution, 20 ml water was added to the solution.

(c) Solution of (b) was added into solution of (a) under continuousstirring for 20 mins.

(d) 4.4 g of NaCl, 0.8 g of NaSnO₃ and 10 g of SnCl₂ were dissolved in50 ml water. Subsequently, solution of (d) was added slowly intosolution of (c) for 30 mins.

(e) The mixture from (d) was kept in a water bath at 65° C. for 4 hoursunder stirring.

Example 2: Characterization and Results

The resultant nickel plated silicone rubber derived from the presentmethod was characterized and the results are discussed as follow.

FIG. 2 shows the water contact angles of neat and TiO₂ coated siliconerubber at 120° and 76°, respectively. With TiO₂ coating and subsequentNaOH etching, the silicone rubber becomes even more hydrophilic withhigher surface energy, having a water contact angle of 28°.

FIG. 3 shows a photo of nickel plated silicone rubber from example 1bwhere the metallic coating is uniform and fully covers the siliconerubber surface.

FIG. 4 shows the scanning electron microscopy (SEM) images of the nickelplated silicone rubber from example 1b. The difference between the leftand right SEM images lies in their magnification, the left image havinga magnification of ×300 while the right image has a magnification of×2500. The plated nickel layer has high continuity, conductivity,smoothness and adhesion to silicone rubber.

The surface conductivity of the nickel plated samples was measured by4-probe resistivity meter (Mitsubishi Chemical Analytech MCP-T370). Thethickness was estimated by the weight of the deposited nickel layer.Adhesion strength between the coating and substrate was determined byusing the pull out adhesion test (DeFelsko PosiTest AT-A) in accordancewith ASTM D 4541. All the data are listed in table 1 below.

TABLE 1 Properties of Nickel Plated Silicone Rubber. Nickel CoatingSurface Adhesion Nickel Thickness Resistivity Strength Deposition (μm)(Ω/sq) (MPa) Example 1b Yes 2.5 1.3 0.48 Example 1c Yes 2.8 0.8 0.79Example 1d Yes 3.0 1.2 0.90 Comparative No — — — Example 1a ComparativeYes 1.4 26.3  0.31 Example 1b Comparative No — — — Example 1c

Example 3: Modification of Silicone Rubber Surface

The structure of TIP is shown below.

TIP can be hydrolyzed by water and this is depicted by the equationbelow.

Ti{OCH(CH₃)₂}₄+2H₂O→TiO₂+4(CH₃)₂CHOH

As the hydrolysis of TIP is fast, TIP is prepared in the form of TIP/IPAsolution before used. Silicone rubber is then immersed into the TIP/IPAsolution. The silicone rubber will be swelled by the IPA and this allowsthe TIP molecules to enter the silicone rubber matrix. Subsequently, thesilicone rubber is removed from the solution and rinsed with IPA. TheTIP inside the silicone rubber matrix then diffuses to the surface andmay be hydrolyzed by moisture in the air to form crosslinked TiO₂ on thesurface of the silicone rubber. This process is shown in FIG. 5.

Apart from using IPA as the swelling agent, other feasible swellingagents may include methanol, 2-methoxyethanol, ethanol, 1-propanol,tert-butanol and/or their mixtures thereof. Apart from coating TiO₂,other transition metal oxide such as zirconium oxide, vanadium oxide,hafnium oxide, niobium oxide or tantalum oxide may be used. This impliesother than TIP as the transition metal oxide precursor, other transitionmetal oxide precursor such as titanium propoxide, titanium methoxide,titanium ethoxide, titanium butoxide, titanium tert-butoxide, titanium2-ethylhexyloxide, zirconium tert-butoxide, zirconium isopropoxide,vanadium isobutoxide, vanadium oxytriethoxide, vanadiumoxytriisopropoxide, vanadium oxytripropoxide, hafnium n-butoxide,hafnium tert-butoxide, niobium ethoxide, tantalum methoxide, tantalumethoxide or tantalum butoxide may be used.

Table 2 below shows how some properties of TiO₂ coated silicone rubbermay be affected by duration of immersion in TIP/IPA.

TABLE 2 Immersion of Silicone Rubber in TIP/IPA Solution (50/50) forVarious Duration before Rinsing with IPA and Drying in Air Time TiO₂Thickness Water Contact Angle (mins) Shore A Hardness (μm) (°) 0 38 0120 1 43 12 85 2 45 19 81 5 49 22 76 10 52 28 74 20 54 33 80

As observed from table 2, the hardness of silicone rubber increased withmore TiO₂ coating. In other words, when the duration of immersion inTIP/IPA becomes longer, the amount of TiO₂ hydrolyzed or coated on thesilicone surface increases and the hardness of the modified siliconebecomes higher. Based on this, the duration of immersion may beconsidered for limiting to 5 mins or less to minimize or avoid too higha hardness while yielding a silicone fully coated with TiO₂. The effectof TIP concentration was also studied, using a silicone rubber withShore A hardness of 30. The results are shown in table 3 below.

TABLE 3 Comparison of TIP Concentration TIP:IPA Water Contact TimeConcentration Shore A Thickness Angle (mins) Ratio Hardness (μm) (°) 525:75 35 5.1 82 10:90 32 2.6 85

As seen from table 3, thickness of TiO₂ decreased when concentration ofTIP was lowered. This also resulted in lowering of hardness. Meanwhile,table 4 below demonstrates the effect of duration of immersion atdifferent concentration of TIP. The concentration ratio is based onvolume of the reagents and hence may be referred to as a volume ratio aswell.

TABLE 4 Effect of Immersion Duration at Different TIP ConcentrationTIP:IPA Water Contact Concentration Shore A Thickness Angle Ratio TimeHardness (μm) (°) 25:75 30 seconds 31 0.9 88 1 min 33 1.6 78 2 mins 343.7 77 10:90 30 seconds 30 0.7 95 1 min 30-31 1.3 84 2 mins 30-31 1.5 81

Example 4: Modifying Hydrophilicity of TiO₂ Coated Silicone Rubber

Neat silicone rubber has a water contact angle of 120° while TiO₂modified silicone rubber may have a water contact angle in the range ofabout 75° to 85°. The decrease in water contact angle signifies thatcoating of silicone rubber with the transition metal oxide, TiO₂, issuccessful. However, the water contact angle of TiO₂ coated siliconerubber is still considered high and not very hydrophilic. It ispostulated that this may be because of adsorption of organic moleculeson TiO₂. While the hydrophilicity of TiO₂ can be tuned using ultravioletradiation, a chemical method is used in the present disclosure instead.

This chemical method relies on using a basic solution to etch thetransition metal oxide, e.g. TiO₂. The basic solution may include NaOH,KOH, LiOH, ammonia etc. An example of using NaOH is represented in theequation below.

TiO₂+2NaOH→Na₂TiO₃+H₂O

As a non-limiting example, this is carried out by immersing the TiO₂coated silicone rubber into 3 M NaOH. After 1 min, the water contactangle was measured, dropping from 74° to 28°. This demonstrates such achemical method is advantageous for enhancing hydrophilicity of thesilicone rubber such that the surface energy of the silicone rubber isincreased.

Example 5: Modification of Etched TiO₂ Silicone Rubber

It was presumed that after etching the TiO₂ silicone rubber, the latterwould have been ready for ENP. To this end, the preparation of a Sn—Pdcolloidal solution was carried out as follows.

(1) Dissolution of 0.1 PdCl₂ in 40 ml 50% HCl (HCl:water in 50:50ratio). The 50% HCl and HCl:water ratio are derived on a volume basis.

(2) 1.0 g of SnCl₂ was added into 20 ml concentrated HCl (37%). Aftercomplete dissolution, 20 ml water was added.

(3) Solution of (2) was added to solution of (1) under stirring. Themixture turned from red to a darker colour under continuous stirring for20 mins.

(4) 4.4 g of NaCl, 0.8 g of NaSnO₃ and 10 g of SnCl₂ were dissolved in50 ml water. Subsequently, solution of (4) was added slowly intosolution of (3) for 30 mins.

(e) The mixture from (4) was kept in a water bath at 65° C. for 4 hoursunder stirring.

It was observed that the resultant Sn—Pd colloidal catalyst (insolution) has a darker colour with no precipitate at bottom. Meanwhile,the composition of an ENP solution was prepared as follows: 4.7 g/l ofNiSO₄, 1 g/l of NaH₂PO₂, 1 g/l of oxycarboxylic acid and a certainamount of ammonia to adjust the pH to 4.9. The ENP solution was green incolour. The etched TiO₂ silicone rubber was then immersed in the Sn—Pdcatalyst solution for 5 mins. It was then removed and rinsed with water,followed by contacting with 1 M HCl for 3 mins to activate the catalyst.Subsequently, the catalyst coated silicone rubber was immersed in theENP solution for nickel electroless plating. The ENP bath was at 88° C.to 90° C. However, few bubbles were observed on the silicone rubbersurface and no reaction occurred. Even after some time, the siliconerubber floated to the surface, signifying that it has turnedhydrophobic, losing its earlier hydrophilicity. This showed that ENP wasunsuccessful. It was then concluded that it was the Sn—Pd catalystdeposition that was not successful because bubbles were actuallygenerated from the metal pin used to load the samples into the ENP bath(see setup in FIG. 7). It was then attributed that both TiO₂ and Sn—Pdcolloidal catalyst being negatively charged (even though both arehydrophilic and possess polar groups), resulted in electrostaticrepulsion leading to unsuccessful deposition of catalyst. A negativelycharged Sn—Pd catalyst is shown in FIG. 6. To resolve this, the etchedTiO₂ silicone rubber surface has to be modified to be positively chargedand branched PEI (from Sigma 408727, Mw about 25,000, Mn about 10,000)was used. Other than PEI, positively charged species such as positivelycharged nanoparticles or nanocolloids, dendrimers comprising nitrogenand/or their mixtures thereof may be used. Polymers having at least oneprimary amine, secondary amine, tertiary amine, pyridinyl, quaternizedamine and/or quaternized pyridinyl and/or their mixtures thereof mayalso be used.

The procedure was therefore changed to immersing the etched TiO₂silicone rubber in 1 wt % PEI solution for 10 mins before contacting thePEI coated silicone rubber with Sn—Pd colloidal catalyst solution, HClactivation solution and the ENP bath which have been described above.The setup of FIG. 7 was used and successful ENP was observed. After PEItreatment and dipping in the Sn—Pd solution, the Sn—Pd deposition can beobserved by the naked eye as a film red in colour on the silicone rubberwas deposited and this red film was not removed when rinsed with water.As for the ENP stage, after immersing the sample into the ENP bath,vigorous bubbling from the sample surface was observed which turneddarker promptly first and then became white grey due to colour of nickeldeposited. A summary of the characterized nickel plated silicone rubberare tabulated in table 5 below. These samples were derived withimmersion duration of 5 mins in TIP/IPA (50:50 volume ratio).

TABLE 5 Characterized Results Summary of Various Samples Conduc-Adhesion Coating Shore A tivity Strength Thickness Hardness SiliconeSample (Ω/sq) (MPa) (μm) 30 From Momentive 26030 1.3 0.48 2.5 70 FromMomentive 22870 0.8 0.79 2.8 43 Dow Corning Slygard 1.2 0.9 3 184

The electrical conductivity of the present nickel plated silicone rubbermay be further improved by, for example, incorporating metal coatedglass beads (about up to 80 wt %), Cu or Ag nanowires, or carbonnanoparticles, into a 2-parts silicone rubber. It can also be improvedwith metallic coating (conductive silicone oil with peroxide, furtherelectroplating etc.) of a 2-parts silicone rubber.

The thermal conductivity of the present nickel plated silicone rubbermay be further improved by, for example, incorporating silicon carbideor boron nitride, into a 2-parts silicone rubber.

Example 6: Potential and Commercial Applications

Application of nickel plated organosiloxane polymers derived through thepresent method can include EMI shielding/gasket, flexible electrodes,soft actuators, microfluidic devices etc.

A comparison of a nickel plated silicone rubber derived via the presentmethod with current industrial products is shown in table 6 below.

TABLE 6 Comparison of Present Sample with Current Industrial ProductsNickel Holland EMI Plated Shielding Conductive Holland Silicone SystemsRubber Shielding Rubber of 5750 ECR-213 Systems 5770 Present (Industrial(Industrial (Industrial Method Product) Product) Product) MaterialNickel Conductive Conductive Nickel plated plated silicone siliconepolyurethane silicone filled with filled with foam rubber Ag/Cunickel/graphite Shore 32 78 60 Less than 10 A Hardness Density 1.1 3.51.9 Less than 0.1 (g/cm³) Resistivity 0.0003 0.002 0.1 0.02 (Ω cm)Working −60 to 220 −60 to 220 −60 to 220 −45 to 85 Temperature (° C.)

Clearly, table 6 demonstrates that the present nickel plated siliconerubber, and the present method, are advantageous over current industrialproducts. It should be noted that lower hardness yields better sealingperformance when the silicone rubber is used for gasket applicationswhile conductive silicone rubber with Shore A hardness below 40 is inhigher demand by the industry. The present nickel plated silicone rubbercan also be developed into a conductive foam.

Apart from using silicone rubber as an example, the present method canbe used to plate nickel on other organosiloxane polymers. One suchexample is PDMS. PDMS is a type of soft mold used in nanoimprinting. Itsadvantages include easy to replicate surface structures, low cost andease of demolding. Accordingly, one potential application of the presentmethod is to imprint metal coated nanostructures using nickel platedPDMS mold. For implantation, the PDMS mold may be first plated withnickel and then used to imprint patterns (e.g. photoresist). This isbecause PDMS has a relatively lower surface energy than thepattern-imprinted material, after demolding, the metal (e.g. nickel) maybe transferred to the material, resulting in a metallized pattern.Unlike e-beam evaporation, the metallized pattern made via the presentmethod can be in any shape and has a continuous metallic surface.

While the invention has been particularly shown and described withreference to specific embodiments, it should be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. The scope of the invention is thusindicated by the appended claims and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced.

1. A nickel organosiloxane polymer composite comprising: a transitionmetal oxide layer formed on the organosiloxane polymer; and a positivelycharged species attached on the transition metal oxide layer with nickelcoated on the positively charged species.
 2. The nickel organosiloxanepolymer composite according to claim 1, wherein the nickelorganosiloxane polymer further comprises a trace amount of Sn—Pdcatalyst under the nickel coated on the positively charged species. 3.The nickel organosiloxane polymer composite according to claim 1,wherein the organosiloxane polymer is selected from the group consistingof polydimethylsiloxane, vinyl methyl polysiloxane, phenyl methylpolysiloxane, phenyl vinyl methyl polysiloxane, fluoro vinyl methylpolysiloxane and derivatives of silicone rubber, wherein thepolysiloxane comprises a quaternary silicon.
 4. The nickelorganosiloxane polymer composite according to claim 1, wherein thetransition metal oxide layer comprises titanium oxide.
 5. The nickelorganosiloxane polymer composite according to claim 4, wherein thetransition metal oxide layer is selected from the group consisting oftitanium oxide, zirconium oxide, vanadium oxide, hafnium oxide, niobiumoxide and tantalum oxide.
 6. The nickel organosiloxane polymer compositeaccording to claim 1, wherein the positively charged species is selectedfrom the group consisting of positively charged nanoparticles ornanocolloids, dendrimers comprising nitrogen, polymers having at leastone primary amine, secondary amine, tertiary amine, pyridinyl,quaternized amine and/or quaternized pyridinyl and their mixturesthereof.
 7. The nickel organosiloxane polymer composite according toclaim 6, wherein the positively charged nanoparticles or nanocolloidscomprise cetyltrimethylammonium bromide (CTAB) stabilized goldnanoparticles, spermidine stabilized silver nanoparticles,2-(dimethylamino)ethanethiol-capped CdTe quantum dots and/or theirmixtures thereof.
 8. The nickel organosiloxane polymer compositeaccording to claim 6, wherein the dendrimers comprise polyamidoaminedendrimers, polyethylenimine dendrimers, polypropylenimine hexadecaaminedendrimers, ammonium-capped thiophosphoryl chloride dendrimers,ammonium-capped cyclotriphosphazene dendrimers and/or their mixturesthereof.
 9. The nickel organosiloxane polymer composite according toclaim 6, wherein the polymers comprise polyamine, polyallylamine,polyetheramine, polyethylenimine, polyvinylpyridine, polybrene,chitosan, poly(2-(trimethylamino)ethyl methacrylate),poly(diallyldimethylammonium chloride),poly(N,N-dimethyl-3,5-dimethylene-piperidinium chloride),poly(2-vinyl-1-methylpyridinium bromide) and/or their mixtures thereof.10. The nickel organosiloxane polymer composite according to claim 2,wherein the nickel, the Sn—Pd catalyst, the positively charged speciesand the transition metal oxide layer comprise a thickness of 2.52 to 3μm.